What is the minimum frequency response which should be represented by an audio codec? Well, what is the best analog or digital system and is it worthwhile to approach, match or exceed its specification?
Analog record players have an exceptionally good minimum frequency response. Technically, the minimum frequency response is determined by the duration of the recording. So, a 45 minute long-play record has a fundamental frequency of 1/2700Hz - which is about 0.0004Hz. Does this make any practical difference? Most definitely yes. People lampoon analog purists or stated that LP and CD are directly equivalent. Some of the lampooning is justisfied but the core argument is true. Most implementations of Compact Disc Audio don't achieve useful functionality.
Many Compact Disc players boldly state "1-Bit DAC Oversampling" or suchlike. The 1-BitDAC is simplistically brilliant and variants are used in many contexts. However, oversampling is the best of a bad bodge. The optics of a Compact Disc are variable and, even with FEC in the form of Reed-Solomon encoding, about 1% of audio sectors don't get read. What happens in this case? Very early CD players would just output nothing. This technique was superceded with duplicate output. (Early Japanese CD players had an impressive amount of analog circuitry and no other facility to stretch audio.)
Eventually, manufacturers advanced to oversampling techniques. In the event that data cannot be obtained optically from disc, gaps are smoothed with a re-construction from available data. Unfortunately, there is a problem with this technique. Nothing below 43Hz can be re-constructed. 2KB audio sectors have 1024×16 bit samples and samples are played at exactly 44.1kHz. So, audio sectors are played at the rate of approximately 43Hz. However, any technique which continues audio waves has a fundamental frequency of 43Hz. Given that drop-outs occur with some correlation at a rate of 1%, this disrupts any reproduction of frequencies below 43Hz. For speech, this would be superior to GSM's baseline AMR which has 50Hz audio blocks. For music, this is a deal-breaker.
Remove the intermittant reading problems and the fundamental frequency is the length of the recording. So, a 16 bit linear PCM .WAV of 74 minutes has a fundamental frequency of approximately, 0.0002Hz. The same data burned as audio sectors on a Compact Disc only has a fundamental frequency of 43Hz. (I'll ignore the reverse of this process due to cooked sectors.)
High Quality Audio, Part 3
What is the minimum frequency response which should be represented by an audio codec? Well, what is the best analog or digital system and is it worthwhile to approach, match or exceed its specification?
Analog record players have an exceptionally good minimum frequency response. Technically, the minimum frequency response is determined by the duration of the recording. So, a 45 minute long-play record has a fundamental frequency of 1/2700Hz - which is about 0.0004Hz. Does this make any practical difference? Most definitely yes. People lampoon analog purists or stated that LP and CD are directly equivalent. Some of the lampooning is justisfied but the core argument is true. Most implementations of Compact Disc Audio don't achieve useful functionality.
Many Compact Disc players boldly state "1-Bit DAC Oversampling" or suchlike. The 1-Bit DAC is simplistically brilliant and variants are used in many contexts. However, oversampling is the best of a bad bodge. The optics of a Compact Disc are variable and, even with FEC in the form of Reed-Solomon encoding, about 1% of audio sectors don't get read. What happens in this case? Very early CD players would just output nothing. This technique was superceded with duplicate output. (Early Japanese CD players had an impressive amount of analog circuitry and no other facility to stretch audio.)
Eventually, manufacturers advanced to oversampling techniques. In the event that data cannot be obtained optically from disc, gaps are smoothed with a re-construction from available data. Unfortunately, there is a problem with this technique. Nothing below 43Hz can be re-constructed. 2KB audio sectors have 1024×16 bit samples and samples are played at exactly 44.1kHz. So, audio sectors are played at the rate of approximately 43Hz. However, any technique which continues audio waves has a fundamental frequency of 43Hz. Given that drop-outs occur with some correlation at a rate of 1%, this disrupts any reproduction of frequencies below 43Hz. For speech, this would be superior to GSM's baseline AMR which has 50Hz audio blocks. For music, this is a deal-breaker.
Remove the intermittant reading problems and the fundamental frequency is the length of the recording. So, a 16 bit linear PCM .WAV of 74 minutes has a fundamental frequency of approximately, 0.0002Hz. The same data burned as audio sectors on a Compact Disc only has a fundamental frequency of 43Hz. (I'll ignore the reverse of this process due to cooked sectors.)
So, it is trivial to retain low frequencies within a digital system. Just ensure that all data is present and correct. Furthermore, low frequencies require minimal storage and can be prioritized when streaming.
A related question is the minimum frequency response for a video codec. That's an equally revealing question.
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